We shall strive more particularly here below in the invention to describe the problems and issues existing in the context of high-broadband electro-optical modulators that have been faced by the inventors of the present patent application. The invention is naturally not limited to this particular field of application but is of interest for any microwave frequency component that has to cope with proximate or similar problems and issues.
Opto-microwave components have in recent years prompted an increasing interest in communications networks requiring wide bandwidths. Indeed, to meet a growing need on the part of users for applications requiring wide bandwidths, communications networks require the use of electro-optical modulation components to transcribe the microwave frequency electrical signals to be transmitted into optical signals. These components have numerous advantages, providing immunity against electromagnetic disturbances, low signal losses, wide bandwidths and low consumption.
Thus, at a time when optics are penetrating high-bit-rate access networks, it appears to be worthwhile to be able to have available high-performance low-cost electro-optical modulators having high integration.
Conventionally, an electro-optical modulator comprises, as illustrated in FIG. 1, a thin layer of dielectric material 1 included between a ground plane 2 (also called a ground electrode) and a microstrip control electrode 3 (also commonly called a microstrip line). The thin layer 1 includes an optical waveguide formed out of electro-optical material 4 disposed so as to guide a light signal to be modulated from an input point up to an output point of the modulator. The microstrip line 3 placed on the upper face of the thin layer 1 and parallel to the optical guide 4 enables the application of an electrical microwave frequency modulation signal intended to modulate the light signal passing through the optical waveguide 4. A phase and/or amplitude modulation of the light signal can be obtained by electro-optical effect. The thin layer 1, based on organic polymers for example, comprises pre-oriented chromophores. The electrical field created inside the electro-optical material 4 by the voltage applied across the control electrode 3 and the ground plane 2 locally modifies the refraction index of the electro-optical material via the electro-optical effect sought.
However, electro-optical modulators made out of thin layers are facing a certain number of obstacles to their technological development.
When an electro-optical modulator has to be connected to a coaxial connector, it is necessary for the outer radius of the conductor of the connector to be of the same magnitude as the thickness of the layer of material forming the substrate 1. Now, owing to the dimensional mismatch between the thin layer 1 (typically of the order of 10 μm) and the conductor of the coaxial connector (typically of the order of several hundreds of micrometers or even several millimeters), a direct connection of the electro-optical modulator with the coaxial connector is ruled out.
In order to avoid any problem of dimensional mismatch and to facilitate the opto-microwave frequency characterizations of the modulators in the non-connected state, classically a probe with coplanar tips (of the GSG or Ground Signal Ground) type is used, requiring the integration of contact pins 5 on the thin layer of the electro-optical material. These contact pins 5 are disposed on either side of the control electrode at input and output of the modulator, as illustrated in FIG. 1.
To convey the electrical modulation signal via the control electrode 3 and the ground pins 5 and apply it to the electro-optical material as efficiently as possible, it is appropriate to have available transitions providing for a virtual electrical contact between the pins 5 and the ground plane 2, by electromagnetic coupling using the capacitive effect between them.
In the prior art, there are different known types of transition in order to improve the electrical bandwidth of an electro-optical modulator
In a first method, metalized holes are introduced into the dielectric substrate, between the ground plane and the contact pins, in order to provide for physical contact between the lower ground plane 2 and the upper ground plane 5. However, this prior-art method is complex in its implementation and unsuited to the making of thin-layer components, especially when the polymer is reticulate. Even when such a method was adopted for making thin-layer microwave frequency components, the presence of metalized holes within the propagation structure generated electromagnetic disturbances, greatly limiting the electrical bandwidth of the component.
A second known method relies on the making of transitions between microstrip lines and coplanar pins by electromagnetic coupling between the lower ground plane and the upper ground planes. In such a structure, when a microwave frequency modulation signal is applied between the contact pins and the control electrode, three modes of electromagnetic propagation can be propagated within the electro-optical material. These three modes of propagation are illustrated in greater detail in FIG. 2: the microstrip mode (denoted MS in the figure), the coplanar mode (denoted as CPW for coplanar waveguide) and the coplanar microstrip mode (denoted as CPM). It has been shown that the use of contact pins in a microstrip type propagation structure greatly excites the CPM propagation mode, which causes the appearance of resonance spikes that limit the electrical bandwidth of the component. In addition, such a method does not ensure the passage of direct current.